Technical: This project aims to investigate fundamental mechanisms governing exciton dissociation at organic-organic or organic-inorganic hybrid semiconductor interfaces. These include energetic driving force, electronic coupling strength, and nuclear reorganization energy in interfacial electron transfer, the potential roles of geminate pair formation, and charge-recombination as competing dynamic channels. Femtosecond time-resolved two-photon photoemission spectroscopy (TR-2PPE) will be used to probe exciton dissociation dynamics at well-ordered organic-organic or organic-inorganic interfaces. The use of single crystal substrates and highly ordered crystalline thin films minimizes heterogeneity problems in nanostructured materials; examples of model systems include oligothiophene and pentacene on single crystal C60 and ZnO surfaces. A TR-2PPE experiment will allow one to directly follow the electron in time, energy, and momentum spaces as it is excited in the form of an exciton in the donor material, as it is en route to the acceptor, as it forms a geminate pair across the donor/acceptor interface, and as it relaxes in the acceptor. The long-term objective is to establish fundamental principles and design rules for exciton dissociation and charge separation at organic heterojunctions for solar cells.
The project addresses basic research issues in a topical area of materials science with high technological relevance, and is expected to fill an important knowledge gap in the development of organic solar cells. Developing solar energy technologies that are economically competitive is one of the most important scientific and technological challenges in this century. From an educational perspective, the project will provide an inspiring opportunity for participating students to work on scientifically challenging and socially responsible research. The PI has established a track record and will continue not only in educating graduate students, but also in motivating and involving undergraduates and high school students in research. The PI will also develop a freshman seminar course which focuses on solar energy and the environment challenges.
This project aimed to explore fundamental photophysical mechanisms that may be utilized to revolutionize future technologies for solar energy conversion. During this funding period, the PI and coworkers discovered a ‘Dark State’ that could mean a brighter future for solar energy. The PI and his team have discovered that it’s possible to double the number of electrons harvested from one photon of sunlight using an organic semiconductor material. Organic or plastic semiconductor solar cell production has great advantages, one of which is low cost. Combined with the vast capabilities for molecular design and synthesis, this discovery opened the door to an exciting new approach for solar energy conversion, leading to much higher efficiencies. Among the ten publications coming out of this research project, one detailed this discovery and its implication for solar energy research [Science 334 (2011) 1541-1545]. The maximum theoretical efficiency of the silicon solar cell in use today is approximately 31 percent, because much of the sun’s energy hitting the cell is too high to be turned into usable electricity. That energy, in the form of so-called "hot electrons," is instead lost as heat. Capturing this wasted energy could increase the efficiency of solar-to-electric power conversion. The PI and his team have found an approach to do that exactly that. They discovered that a photon produced a dark quantum "shadow state" from which two electrons could then be efficiently captured to generate more energy in the organic semiconductor pentacene. Exploiting that mechanism could increase solar cell efficiency to 44 percent theoretically.
